When you have these symptoms:
Mac computer repeatedly prompts for password but small print is about maximum number of connection from user+IP exceeded mail_max_userip_connections=10
This error can be witnessed in /var/log/mail.log
tail -f /var/log/mail.log | grep boss Mar 14 18:19:28 hostname dovecot: imap-login: Maximum number of connections from user+IP exceeded (mail_max_userip_connections=10): user=<[email protected]>, method=PLAIN, rip=a.b.c.d, lip=e.f.g.h, TLS, session=<7nZyCzDaEs0Yudk2>
How to refine the process of isolation
grep -r "mail_max_userip_connections" /etc/dovecot /etc/dovecot/conf.d/20-imap.conf: #mail_max_userip_connections = 10 /etc/dovecot/conf.d/20-pop3.conf: #mail_max_userip_connections = 10
grep -r is your new best friend
Solution
[[email protected]:/etc/dovecot]> vi conf.d/20-imap.conf
Uncomment that line and change the default. Be conservative, for example, make 10 to be 15. Restart dovecot
Comment
This only appeared to happen on one Macintosh computer out of 1000+ email accounts. It seems the Mac (or that Mac) occassionally overwhelmed the Dovecot server with more than 10 connections from the same IP at once. This seems like a networking error but to preserve the peace (and have less ‘urgent’ customer services) we upped the limit.
Reference